Reinforced pavements



May 6, 1958 c. DOBELL REINFORCED PAVEMENTS original Filed March 11, 195sREINFORCED lPAVEMENTS Curzon Dobell, Nassau, Bahamas, assignor to ThePreload Company, Inc., New York, N. Y., a corporation of Delaware iContinuation of application Serial No. 341,770, March 11, 1953. Thisapplication November 19, 1956,Serial No. 622,896

3 Claims. (Cl. y94-4--Z2) This invention relates to the construction ofconcrete surfaces resting on a sub-base, such as pavements or road,ways, and more particularly. to theY construction of concrete roadwaysthat are held under continuouslongitndinal compression in such a waythat cracking and buckling are avoided and expansion joints becomeunnecessary.

At present, in particular in designing runways forV heavy bombers,engineers `are faced with the problem of using either Iabnormally thickvand expensive slabs of reinforced concrete to spread the load over thesub-base suiciently so that little or no deformation will occur, ortrying to nd something such as asphaltic materials which will deformunder the wheel loads and return to normal levels after the load hasrolled on. To date, pref stressed concrete is the only medium'whichoffers both an economical slab thickness which can sustain the loadings,and a material which has elasticity enough to deform under the wheelsand return to normal when the load has passed.

The common diiculties encountered in concrete roads a time with theprestressed reinforcing wires or rods running lengthwise and crosswiseof the section, ordinary cracking may be prevented. However, expansionjoints is another object to create a pavement which would be l acontinuous slab having a uniform concrete section in compressive stressand a uniform steel section in tensile stress, both over a substantiallyunlimited length, with the elastic shortening caused by the compressingof the slab absorbed in increments by the use of temporary constructionjoints. vIt is yet another object to avoid and eliminate costlyexpansion joints -between sections of pavement. And it is yet a furtherobject to provide an easily laid roadway notV requiring elaboratepreparation of the,

reinforcing elements. It has been found that this can be done by themethod and arrangement set forth in this application and that as aresult cracking-and buckling.

may be avoided.

lBrietly, this invention consists in prestressing a concrete road 'bydiagonally placed elements so that the ends of the prestressing elementscan be tensioned from the edges of the slab and simultaneously providinga means and method to take up the longitudinal distance that theconcrete of the road loses by reason of its being placed underlongitudinal compression, by providing temporary lconstruction jointsthat are later-filled, allowed to set and compressed.

The result is a continuous concrete pavement vunderv y sucha mannerthatthere is no tendency of the roadf to buckle or crack even whenthelongitudinal compression a is increased by increases in temperaturethat are restrained from causing corresponding increases in length.

It is preferred,.in applying the principles of this inven-A tion, -touse `prestressing material of a quality such that relatively hightension may be applied to it so that after the concrete has undergoneany plastic flow of which it is capableand the material of theprestressing element has taken on any permanent elongation that it willtake on as a'result of the forces bearing upon it, there will still be ahigh degree of tension in the. prestressing elements and a high degreeof compression in the concrete.

An understanding'of both the details and the principles of thisinvention may best be had by an immediate ref- --ierence to thedrawingswhich illustrate the preferred mode are still necessary betweenthe sections to compensate t for changes in length of the sections dueto changes in temperature and ordinarily these must be made quite widein 4order both to provide for access to the ends of the longitudinalwires or rods so that they may be tight-Y tions as a whole undercompression, and at the joint bein-accordance with the principles ofthis invention asv4 tween two adjacent sections serious bucklingeouldgoccur. In the use of longitudinal stressing the length of thesection is determined by the loss of prestress between is increasedrequiring elaborate precautions yto accommodate the increased movement.

"'pavement constructed in accordance with this invention; f

of application of the principles of this invention and to the followingdetailed description of lthis mode of application of this invention. l

In the drawings, Figure l is a plan view of a concrete while Figure 2shows thel same in more detail;

Figure 3 is a cross section of Figure 2 taken along line 3 3; and

The object of the present invention is to provide a syse* tem ofstressing pavements in which the slab is stressed f` continuously alonglits length and when possible continuously along its breadth without anyinterruptions in the stressing reinforcing for expansion joints or 'thelike. It

Figure 5 is a detail .view of the end anchorage for the reinforcingrods.

Figure. 6 is a diagram showing the slab compression under variousconditions of compression.

Pavements or slabs on any subbase maybe constructed extendingreinforcing elements 12 and 13, which are coated to prevent bonding withtheconcrete or placed in preformed holes. Balanced stresses areprovidedby inclining one series of elements 12 oppositely from Vthe`other series 13. It is necessary that the ends of the ele- '-ments bearranged in a method to avoid interference with the tensioning device. Arecess 14 is provided at each end ofeach of these elements so as toprovidel access to it for tensioning the element and a nut 15 or otherappropriate means for holding the element intenysion isalso provided.The detail in Figure 5 shows the atented May 6, 1958 Figure 4 is a crosssection of Figure 2 taken along line I end anchorage which is a threadedbarl over which is placed a bearing plate 25 followed by a washer 26 andiinally the nut 15.` A suitable `bearing plate -for a l 4inch bar wouldbe steel plate 11/2 inches thick and 5 inches by 6 inches` in area. Anyothertype of, end anchorage adaptedito secure the elements will -beadequate. The concrete may be prevented from bonding to the elements inany of the known ways. For example, the elements may be `covered withasphalt before the concrete l is molded around them or they may beplaced in preformed holes in the concrete. e

As can be seen from Figure 2, a concrete roadway is cast in a series ofspaced sections 10 and 11 but the reinforcing elements extend acrossthefspace between the sectionsiso that some of the reinforcing elementslie partly in one section and partly in another.

After the concrete has set, the elements 12 and 13 are tensioned toplace the concrete under both lateral and longitudinal compression. As aresult of their compression it is natural that the length and width ofeach concrete section will diminish to a certain extent by elasticcontraction.` The length of a A500 ft. road section, for example, willdecrease some three or four inches.

This `invention contemplates that in order that this Vde-` crease inlength takes place from the ends of the section toward the center andthat it be compensated,appropri ate instrument for forcing the endsapart, such as a hydraulic jack shownV schematically at 20 is providedso that it will lit between the abutting ends of the adjacent sectionsand force their ends apart. A pump 21 is connected to the jack, and thejack, which consists of an elongated pressure chamber that isflexible sothat it can expand, is put under pressure and presses against theabutting ends of the adjacent sections so as to force them apart astension is placed on the reinforcing material, The abutting ends of theadjacent sections may be made somewhat thicker to form a haunch 23 asshown in Figure 3 so as to properly accommodate this hydraulic jack.

After the tension has been placed on` the reinforcing elements and theconcrete placed under compression by the combined action of this tensionand the pressure of the `hydraulic jack 20, the space above thehydraulic jack 20V is filled with concrete or other suitable material 22as shown in Figures 3 and 4. The pressure may be released in thehydraulic jack 20 and the jack withdrawn after the material filling thejoint has hardened sutiiciently. The compression in the concrete then istransferred against the material filling between the sections and thewhole section is placed under substantially equal compression.

In my scheme the jacks and tensioned reinforcing serve entirelydifferent purposes. The` concrete is placed under the requiredcompression by the post-tensioning of the reinforcing. Bypost-tensioning is meant the stressing v of the elements after theconcrete has set. However, it would be impossible to prestress apavement for any great length -by post-tensioning the reinforcing alonewithout providing for some periodic adjustment of the elastic strain inthe concrete due` to prestressing. This adjust- `mentwould amount tosome 3.5 ft. per mile at a 300 out interrupting the continuity of therods passing through p. s; i. stress. This movement is resisted byfriction between the pavement and the sub-base.

To absorb the elastic strain in the concrete due to prestressing,temporary construction joints are provided at intervals of suchfrequency (between 200 and 500 ft. depending on subsoil frictionvalues), so as to reduce the frictionloss between joints to a reasonableminimum. Jacks are placed between the vertical surfaces of the concreteat these joints. The prestressing is preferably placed diagonally in theslab, probably in the form of high strength bars, at any desired angleto the axis of the pavement which provides the desired ratio oflongitudi. nal and transverse stress. These bars are coated so that theywill act as sliprods to avoid the. cost of subsequent grouting, or maybeplaced inspreforrnedholes.Y

As the tensioning of the reinforcing progresses along the pavement thejacks are placed in the construction joints and are actuated to increasethe gap in the joint, as measured by micrometers, by the amount of theelastic shortening of the concrete in the length of this joint. If thejoint as cast was l inch wide and the elastic shortening in a 500 ft.slab 0.35 inch, the jack would be used to enlarge the joint from 1 inchto 1.35 inches as the prestressing progressed across the joint.

The jacks would then be used to temporarily increase the elasticshortening (such as to increase the joint to 1.7 inches), `which wouldincrease the compression in the concrete at the joint for a shortperiod, and thus reduce the frictional loss at the mid-point between thetwo joints. The gap would then be relaxed back to the 1.35 inches, atwhich gap spacing the joint would be grouted with mortar. After themortar has retained sufficient strength'the jack would be removed, thustransferring the `full force of the pre-tensioned steel across themortared joint, and the jacks would be reused at a subsequent joint. Bythe use of temporary over-stressmg as described, 'the mid-point betweenthe joints would vreceive an increased compression, which would be atleast partially retained afterthe jack was removed, resultmg m a moreuniform distribution of compression.

This is shown in the diagrammatic sketch of Figure 4 wherein theconcrete roadbed r has a length l. If the Vcompression which is normallyexpected to be maintained is applied tol the roadway, the maximumcompresslon will be realized at the ends of the slab while there will besubstantially no compression at the mid-point of the slab. This is shownin line a with the maximum 'compression d at the slab end. lf double thecompression is applied, the compression over the slab will beexemplitied by linefb, showing the increment y due to the extracompression, and further showing the compression now existing at themid-point of the slab. Reducing the gap or the compression back to thenormally expected standard results in line c, wherein a more evenlysmoothed olutbcompression results across the entire length of the s a Ithas .been found preferable, in this construction, to use steel wire orrods or other suitable materialshaving a tensile strength of at least150,000 pounds per square inch, and to tension wires or rods initiallyto a tension of atleast 100,000 Vpounds per square inch. It ispreferable,but not entirely necessary, to use concrete for filling thejoints,of,the same type as that use in the slab in order-to 4have auniform material throughout.

Adiscussion of the h'aunch 23 is in order. By providing a thickenedportion of the concrete slab there is provided an adequate bearingsurface for the jack 20 withthe gap between the slabs, and withoutinterrupting the continuity ofthe concrete poured in the gap to Vtill itafter the jack has forced the sections apart. When this concrete has setand the jack tension is removed, the

vconcrete iniwthe gap is placed under the same compression as the restof the pavement, `and by the use ofthe haunch, the concrete in the gapis the same thickness as 1nthe rest of the pavement.v An alternative tothe v haunch isthe use of a series ofnotches in the ends of the f slabsat the joints vto retain 'a jack set on top of the slab,

and, having the concrete under a relatively uniform compression and theprestressing elements under a relatively uniform tension, is in acondition where it is quite stable and there is little or no tendency tobuckle or crack even under relatively wide changes in temperature.

Any tendency to buckle is resisted by the prestressing elements. Asthese are continuous along the enti-re length of the roadway, there areno weak points or interruption of the section at which buckling mayoccur.

Many modifications and improvements that will be apparent to thoseskilled in the art are considered to be within the scope of the appendedclaims.

This application is a continuation of my application Serial No. 341,770,tiled March ll, 1953, now abandoned,

I claim:

l. The method of constructing a long concrete slab resting on a sub-basewhich comprises forming a series of slab sections between whichtemporary transverse joints are provided at intervals, incorporating inthe sections and across the joints high strength reinforcing elcments inunbonded engagement with the sections, permitting the sections to setuntil they have acquiredconsiderable strength, tensioning saidreinforcing elements, increasing the width of the temporary joints byforcing them apart with an expanding means to an amount corresponding tothe `amount of elastic shortening which would occur in the sections ifunrestrained by -friction with the sub-base, lling the extender jointswith a material capable of attaining sucient strength to resistsubstantial compression and removing the expanding means after the jointmaterial has attained such strength to transfer compression from thesections to the joint material.

2. The method of constructing a long concrete slab resting on a sub-basethat comprises forming `a series of sections between which temporarytransverse joints are provided at intervals, incorporating in thesections and across the joints high strength reinforcing elementsextending diagonally across the joints and through the side edges of thesections, said reinforcing elements being in unbonded engagement withthe section, permitting the sections to set until they have acquiredconsiderable strength, separately tensioning said reinforcing elementsto provide a predetermined compression in the sections causing anelastic shortening of the sections, increasing the width of thetemporary joints by an amount equal to the elastic shortening of thesections at the joints while tensioning is in progress by forcing themapart with an expanding means, filling the joints while in the enlargedposition with material capable of achieving a set of suiiicient strengthto resist substantial compressive forces, and relaxing the expandingmeans after the joint material has set so as to transfer the compressionfrom the sections to the joint material.

3. A method as defined in claim 2, with the addition of forcing thesections apart a-t a pressure in excess of that required to create theultimately desired compression in the roadway, holding them at theincreased pressure a sufficient time to permit the compression to bedistributed to the center of the sections, partially releasing thecompression before lling the space between the sections With concrete.

No references cited.

